1. Field of the Invention
This invention relates to a Cu—Ti-based copper alloy sheet material suitable for use in electrical and electronic parts such as connectors, lead frames, relays, switches and the like, particularly to the copper alloy sheet material that exhibits excellent bending workability and stress relaxation resistance while maintaining high strength, and to a method of producing the same.
2. Background Art
Materials for use for components such as connectors, lead frames, relays, switches and the like that constitute electrical and electronic parts require high “strength” capable of enduring stress imparted during assembly and/or operation of the electrical or electronic parts. Because electrical and electronic parts are generally formed by bending, they also require excellent “bending workability”. Moreover, in order to ensure contact reliability between electrical and electronic parts, they require endurance against the tendency for contact pressure to decline over time (stress relaxation), namely, they need to be excellent in “stress relaxation resistance”.
Of particular note is that as electrical and electronic parts have become more densely integrated, smaller and lighter in weight in recent years, demand has increased for thinner copper and copper alloy materials for use in the parts. This in turn has led to still severer requirements for the level of “strength” of materials. To be more specific, a strength level expressed as tensile strength of 800 MPa or greater, preferably 900 MPa or greater, even more preferably 1000 MPa or greater, is desired.
Further, the emergence of smaller and more complexly shaped electrical and electronic parts has created a strong need for improved shape and dimensional accuracy in components fabricated by bending. The importance in the requirement for “bending workability” includes not only the absence of cracks in the bent areas but also ensured shape and dimensional accuracy of the articles worked by bending. A troublesome problem occurring more or less in bending is spring-back. Spring-back is a phenomenon of elastic deformation recovery of a worked article taken out of a mold, which means that the shape of the article taken out of a mold differs from that of the article just after worked in the mold.
With the increase in the requirement for the strength level of materials to a further higher degree, the problem of spring-back tends to increase. For example, in fabricating connector terminals having a box-like bent shape, the shape and the dimension of the terminals may be out of order owing to spring-back, and they may be after all useless. Recently, therefore, increased use is being made of a bending method in which the starting material is notched at the location to be bent and bending is later carried out along the notch (hereinafter referred to as “notch-and-bend method”). With this method, however, the notching work hardens the vicinity of the notch, so that cracking is apt to occur during the ensuing bending. The “notch-and-bend method” can therefore be viewed as a very harsh bending method from the viewpoint of the material.
In addition, the fact that more and more electrical and electronic parts are being utilized in severe environment applications has made “stress relaxation resistance” as an increasingly critical issue. For example, “stress relaxation resistance” is of particular importance when the part is exposed to a high-temperature environment as in the case of an automobile connector Stress relaxation refers to the phenomenon of, for instance, a spring member constituting an element of an electrical or electronic part experiencing a decline in contact pressure with passage of time in a relatively high-temperature environment (e.g., 100 to 200° C.), even though it might maintain a constant contact pressure at normal temperatures. It is thus one kind of creep phenomenon. To put it in another way, it is the phenomenon of stress imparted to a metal material being relaxed by plastic deformation owing to dislocation movement caused by self-diffusion of atoms constituting the matrix and/or diffusion of solute atoms.
But there are tradeoffs between “strength” and “bending workability”, or between “bending workability” and “stress relaxation resistance”. Up to now, the practice regarding such current-carrying components has been to take the purpose of use into account in suitably selecting a material with optimum “strength”, “bending workability” or “stress relaxation resistance”.
A Cu—Ti-based copper alloy has high strength next to a Cu—Be-based alloy of copper alloys, and has stress relaxation resistance over a Cu—Be-based alloy. From the viewpoint of the cost and the load to the environment thereof, a Cu—Ti-based alloy is superior to a Cu—Be-based alloy. Accordingly, a Cu—Ti-based copper alloy is used for a connector material as a substitute for a Cu—Be-based alloy. However, it is generally known that, like a Cu—Be-based alloy, a Cu—Ti-based alloy is an alloy system capable of hardly satisfying both “strength” and “bending workability”.
Accordingly, in many cases, a Cu—Ti-based alloy sheet material is shipped while it is still relatively soft before aging treatment, and then, after shaped by bending and/or pressing, it is hardened by aging treatment. However, the method of aging treatment after bending and/or pressing is disadvantageous for producibility improvement and cost reduction since the worked alloy may be discolored owing to oil adhesion thereto and since the method requires an exclusive furnace for heat treatment. Accordingly, of Cu—Ti-based copper alloy sheet materials, market needs are increasing these days for sub-aged materials (mill-hardened materials) that do not require aging treatment after bending and/or pressing. Mill-hardened materials are sheet materials that have been aged to a level not reaching the maximum hardness thereof. The advantage of using them is that the aging treatment after working into parts may be omitted in many applications not requiring the maximum strength level. However, though relatively light, it cannot be denied that the sub-aging treatment may worsen the workability of the materials.
In general, refinement of crystal grain size effectively improves “bending workability”, and the same shall apply to a Cu—Ti-based copper alloy. However, the crystal grain boundary area per unit volume increases with decreasing the crystal grain size. Accordingly, crystal grain refinement promotes stress relaxation, which is a type of creep phenomenon. In relatively high-temperature environment applications, the diffusion velocity of the atom along grain boundaries is extremely higher than that inside the grains, so that the loss of “stress relaxation resistance” caused by crystal grain refinement becomes a major problem.
Further, in a Cu—Ti-based copper alloy, “precipitates” exist essentially as an intragranular modulated structure (spinodal structure), and there are a relatively few “precipitates” to be the second phase grains acting for pinning the growth of recrystallized grains; and during the step of treatment for solid solution formation, it is not easy to attain crystal grain refinement.
In recent years, crystal grain refinement and control of crystal orientation (texture) have been proposed for improving the properties of Cu—Ti-based alloys (see Patent References 1 to 4).    Patent Reference 1: JP-A 2006-265611    Patent Reference 2: JP-A 2006-241573    Patent Reference 3: JP-A 2006-274289    Patent Reference 4: JP-A 2006-249565
It is well known that crystal grain refinement and control of crystal orientation (texture) are effective for improving the bending workability of copper alloy sheet materials. Regarding control of the crystal orientation (texture) of a Cu—Ti-based copper alloy, in the case where ordinary production processes are utilized, the X-ray diffraction pattern from the sheet surface (rolled surface) is generally dominated by the diffraction peaks from the four crystal planes {111}, {200}, {220} and {311}, and the X-ray diffraction intensities from the other crystal planes are very weak compared with those from these four planes. The diffraction intensities from the {200} plane and the {311} plane are usually large after solution heat treatment (recrystallization). The ensuing cold rolling lowers the diffraction intensities from these planes, and the X-ray diffraction intensity from the {220} plane increases relatively. The X-ray diffraction intensity from the {111} plane is usually not much changed by the cold rolling.
In Patent Reference 1, the cold rolling ratio before solution heat treatment is defined to be at least 89% for crystal grain refinement. The strain introduced at such a high rolling reduction ratio functions as a nucleus for recrystallization, thereby giving fine crystal grains having a grain size of from 2 to 10 μm or so. However, the crystal grain refinement of the type is often accompanied by reduction in “stress relaxation resistance”. In addition, since the hot-rolling temperature is 850° C. and is high, the technique of this reference could not sufficiently improve the bending workability of the alloy, as so confirmed by the present inventors' investigations.
Patent Reference 2 defines the X-ray diffraction intensity ratio from {220} and {111}, as I{220}/I{111}>4, for improving the strength and the conductivity of the alloy. This kind of texture regulation to define the {220} plane as the main orientation component may be effective for improving the strength and the conductivity of the alloy, but lowers the bending workability thereof, as so confirmed by the present inventors' investigations. In fact, Patent Reference 2 is silent on the bending workability of the alloy.
Patent Reference 3 proposes a texture of an alloy having improved bending workability of such that, in the {111} pole figure thereof, the maximum value of the X-ray diffraction intensities within the four regions including {110}<115>, {110}<114> and {110}<113> is from 5.0 to 15.0 (in terms of the ratio to the random orientation). For obtaining the texture of the type, the cold-rolling reduction ratio before the solution heat treatment is defined to be from 85 to 97%. The texture of the type is a typical alloy-rolled texture ({110}<112> to {110}<100>), and its {111} pole figure is similar to the {111} pole figure of 70/30 brass (for example, see “Metal Data Book”, 3 Rev. Ed., p. 361). According to the conventional method of controlling the crystal orientation distribution on the basis of the alloy texture, it is difficult to significantly improve the bending workability of alloy. In fact, the bending workability in Patent Reference 3, R/t is at most 1.6.
Patent Reference 4 proposes an alloy texture satisfying I{311}/I{111}≧0.5. However, the present inventors' investigations confirmed that it is difficult to stably and remarkably improve the bending workability of the alloy of the type.
Use of the above-mentioned notch-and-bend method on a copper alloy sheet material effectively improves the shape and dimensional accuracy of the bent article. However, in the Cu—Ti-based alloys having the controlled texture as in Patent References 1 to 4, no consideration is given to preventing cracking caused by the notch-and-bend method. The present inventors' investigations confirmed that the bending workability after notching of the alloys is not sufficiently improved.
Cu—Ti-based alloy sheet materials are often supplied as mill-hardened materials, but the mill-hardened materials are problematic in that the bent articles thereof could hardly maintain the shape and dimensional accuracy because of spring-back. For spring-back reduction, the above-mentioned “notch-and-bend method” may be effective, but in the working method, the area around the notched part is work-hardened owing to notching, and therefore it may be readily cracked during the ensuing bending. At present, the “notch-and-bend method” is not as yet industrially employed for mill-hardened materials of Cu—Ti-base alloys.
Further, as so mentioned in the above, crystal grain refining may be effective in some degree for improvement of bending workability, but on the contrary, it is a negative factor in overcoming stress relaxation, a type of creep phenomenon. From these, only for the “bending workability”, its high-level improvement is difficult in the current situation, and further improvement of “stress relaxation resistance” could not be realized even though known texture control techniques are utilized.